Stool samples frequently must be prepared for medical diagnostic analysis. Stool samples may be analyzed for diagnosis of medical conditions ranging from parasitic, bacterial or viral infections to inflammatory bowel disease and colorectal cancer.
Colorectal cancer is a leading cause of death in Western society. However, if diagnosed early, it may be treated effectively by removal of the cancerous tissue. Colorectal cancers originate in the colorectal epithelium and typically are not extensively vascularized (and therefore not invasive) during the early stages of development. Colorectal cancer is thought to result from the clonal expansion of a single mutant cell in the epithelial lining of the colon or rectum. The transition to a highly vascularized, invasive and ultimately metastatic cancer which spreads throughout the body commonly takes ten years or longer. If the cancer is detected prior to invasion, surgical removal of the cancerous tissue is an effective cure. However, colorectal cancer is often detected only upon manifestation of clinical symptoms, such as pain and black tarry stool. Generally, such symptoms are present only when the disease is well established, and often after metastasis has occurred. Early detection of colorectal cancer therefore is important in order to significantly reduce its morbidity.
Invasive diagnostic methods such as endoscopic examination allow for direct visual identification, removal, and biopsy of potentially cancerous growths. Endoscopy is expensive, uncomfortable, inherently risky, and therefore not a practical tool for screening populations to identify those with colorectal cancer. Non-invasive analysis of stool samples for characteristics indicative of the presence of colorectal cancer or precancer is a preferred alternative for early diagnosis, but no known diagnostic method is available which reliably achieves this goal.
Current non-invasive screening methods involve assaying stool samples for the presence of fecal occult blood or for elevated levels of carcinoembryonic antigen, both of which are suggestive of the presence of colorectal cancer. Additionally, recent developments in molecular biology provide methods of great potential for detecting the presence of a range of DNA mutations or alterations indicative of colorectal cancer. The presence of such mutations can be detected in DNA found in stool samples during various stages of colorectal cancer. However, stool comprises cells and cellular debris from the patient, from microorganisms, and from food, resulting in a heterogeneous population of cells. This makes detection of small, specific subpopulations difficult to detect reliably.
Use of the polymarase chain reaction (PCR) has made detection of nucleic acids more routine, but any PCR is limited by the amount of DNA present in a sample. A minimum amount of material must be present for specific analysis and this limitation becomes more relevant when one seeks to detect a nucleic acid that is present in a sample in small proportion relative to other nucleic acids in the sample, which is often the case when analyzing stool sample for detecting DNA characteristics of colorectal cancer. If a low-frequency mutant strand is not amplified in the first few rounds of PCR, any signal obtained from the mutant strand in later rounds will be obscured by background or by competing signal from amplification of ubiquitous wild-type strand.
An additional problem encountered in preparation of stool sample for detection of colorectal cancer is the difficulty of extracting sufficient quantities of relevant DNA from the stool. Stool samples routinely contain cell debris, enzymes, bacteria (and associated nucleic acids), and various other compounds that can interfere with traditional DNA extraction procedures and reduce DNA yield. Furthermore, DNA in stool often appears digested or partially digested, which can reduce the efficiency of extraction methods.